1. Field of the Invention
The present invention relates generally to a method and apparatus for determining transmission power of a mobile station in a mobile communication system, and in particular, to a method and apparatus for determining initial transmission power of a mobile station in an Orthogonal Frequency Division Multiplexing (OFDM) system.
2. Description of the Related Art
Recently, in the mobile communication system, extensive research has been conducted on an OFDM scheme for use in high-speed data transmission over a wire/wireless channel. The OFDM scheme, for transmitting data using multiple carriers, is a type of Multi-Carrier Modulation (MCM) scheme that converts a serial input symbol stream into parallel symbol streams and modulates each of them with multiple orthogonal sub-carriers, i.e., multiple sub-carrier channels, before transmission.
The OFDM scheme is widely applied to digital transmission technologies such as Digital Audio Broadcasting (DAB), Digital Video Broadcasting (DVB), Wireless Local Area Network (WLAN) and Wireless Asynchronous Transfer Mode (WATM). In particular, as the OFDM scheme overlaps frequency spectra, it is efficient in frequency utilization, robust against frequency-selective fading and multi-path fading, effective in reducing an Inter-Symbol Interference (ISI) effect using a guard interval, and simple as to hardware design of the equalizer structure. In addition, since the OFDM scheme is robust against impulse noises, it can be positively used for communication systems.
In the OFDM scheme-based mobile communication system (hereinafter OFDM system), a Forward-link Shared Control Channel (F-SCCH) is for transmitting a message related to forward/reverse resource assignment and management, format definition of data packet, and access attempt authentication of a mobile station. In order for the mobile station to actually transmit data in the OFDM system, control information transmission over the F-SCCH should be achieved. The longest-length control channel message transmitted over the F-SCCH has, for example, a 25-bit length.
Among the multiple control channel messages transmitted over the F-SCCH, a Reverse-Link Assignment Message (RLAM), which is used for assigning or releasing the resources that the mobile station should use in the reverse link, includes fields for (i) a characteristic (e.g., indicating whether the corresponding resource is persistent assignment resource) of resources assigned to a mobile station, (ii) a Packet Format (PF) for defining a Modulation and Coding Scheme (MCS), (iii) a pilot pattern, and (iv) a Reverse-Link Power Control (RLPC).
Table 1 shows some of the fields constituting the RLAM in the conventional mobile communication system.
TABLE 1BlockMACChanRLExt.FieldtypeIDPersistentIDPilotPFTXRLPC# bits49-11161415RLAM00010111101
In Table 1, Block type=‘0001’ indicates the RLAM, a field value ‘0’ indicates an inactive state of the corresponding field and a field value ‘1’ indicates an active state of the corresponding field. The total number of bits that the base station needs to transmit the RLAM is 25.
The RLPC field is carrier-to-interference Power Spectral Density (PSD) information for the data provided to the mobile station so that the mobile station may determine an initial transmission PSD that it should use when transmitting data using the reverse resources assigned from the base station, and the carrier-to-interference PSD information for the data will be referred to herein as ‘DataCtoIassigned’. The DataCtoIassigned, as defined in Equation (1), is applied for determining a transmission PSD increment ΔLX of the mobile station.ΔLX=IoT−pCoT+DataCtoIassigned  (1)
In Equation (1), IoT denotes an Interference-over-Thermal PSD that the base station receives from adjacent cells, and pCoT denotes a Carrier-over-Thermal PSD for a pilot. These PSD values can be transmitted over a separate control channel.
When several possible PFs are considered, if a scope of DataCtoIassigned required in the general level is assumed to be, for example, −5 dB to 25 dB, and the scope of the DataCtoIassigned is divided by a magnitude of 1 dB, a total of 31 power levels are generated, so 5 bits are needed to express the power levels. For example, when 5-bit information of ‘00001’ to ‘11111’ is used to express the power levels, the mobile station, upon receiving ‘00001’ from the base station, determines −5 dB as a required reception DataCtoIassigned, and uses 8 dB for ‘01101’. Therefore, the mobile station reads the 5-bit information in the RLPC field, and determines a level of the transmission PSD, with which it will transmit data, depending on the read information.
FIG. 1 illustrates generally a method for determining reverse transmission power of a mobile station in a conventional OFDM system, and in particular, a mobile station's operation of determining transmission power of data using a RLPC field of an RLAM.
Referring to FIG. 1, in step 101, a mobile station receives from a base station an F-SCCH over which an RLAM is transmitted. Upon detecting successful demodulation of the F-SCCH in step 103, the mobile station proceeds to step 105 where it reads 5-bit information as a field value of an RLPC field from the RLAM. Thereafter, in step 107, the mobile station determines a level of initial transmission power according to a DataCtoIassigned value corresponding to the RLPC field.
Since the above-described prior art allows the mobile station to determine transmission power it will use, after reading the RLPC field inserted in the RLAM, and considering only the RLPC field in determining the level of the transmission power, the required amount of information of the RLPC field increases with the number of power levels that the base station desires to support. Therefore, according to the prior art, the size of the RLPC field may be larger than necessary in the RLAM.